The present invention concerns a reflective liquid crystal display device in particular with improved display contrast allowing the displayed characters to contrast clearly with the background on which they are displayed.
There exist different types of reflective liquid crystal display devices. One may, in particular, cite transmissive display devices including a metal or dielectric reflector placed behind the display cell.
A transmissive display device conventionally includes an absorbent polariser placed in front of the cell, and a second absorbent polariser placed behind the cell and whose direction of polarisation is perpendicular to that of the first absorbent polariser placed in front of the cell. Such is the case, in particular, of twist nematic or TN liquid crystal cells. A first linear component of the light is absorbed by the absorbent polariser placed in front of the cell, i.e. on the side receiving the incident light, while the second light component, perpendicular to the first, is transmitted to the TN cell. In the TN cell, the direction of polarisation of the transmitted light is rotated through 90xc2x0, so that this light is transmitted without modification by the second absorbent polariser, and is visible to an observer placed behind the cell. In the zones where a control voltage is applied in order to modify the optical conditions of the twist nematic liquid crystal, the light transmitted by the absorbent polariser placed in front of the cell passes through said cell without being modified by the TN liquid crystal and is absorbed by the second absorbent polariser placed behind the cell. The observer, placed behind the cell, thus sees characters displaced in black on a white background.
In order to transform a transmissive cell into a reflective cell, as mentioned above, one need only arrange a metal or dielectric reflector behind said transmissive cell. Thus, in the event chat the display cell is a TN liquid crystal cell, the linear component of the light transmitted to the TN cell by the absorbent polariser placed in front of said cell is rotated through 90xc2x0 by the liquid crystal, then transmitted without modification by the second absorbent polariser placed behind the TN cell whose direction of polarisation is perpendicular to that of the first absorbent polariser, then finally reflected by the reflector. The light then follows the same path, but in the opposite direction, through the cell and the absorbent polarisers, and is visible to an observer placed in front of said cell, i.e. on the side through which the incident light penetrates.
There also exist reflective liquid crystal cells without any polarisers. This is the case, for example, of polymer stabilised cholesteric texture or xe2x80x9cPSCT xe2x80x9d liquid crystal cells.
It will be recalled that the characteristic of certain cholesteric type liquid crystals is having a periodic twisted structure which reflects, by Bragg reflection, light which has circular polarisation in the same rotational direction as that of the liquid crystal. Thus, a cholesteric liquid crystal having a right handed helix will reflect light circularly polarised to the right, whereas it will allow light circularly polarised to the left pass without modification. Conversely, a cholesteric crystal having a left handed helix will reflect light circularly polarised to the left.
Display cells without any polarisers which use reflective liquid crystals such as cholesteric liquid crystals require the use of an absorbent black layer conventionally deposited on the back face of the cell. When a suitable control voltage is applied-to selected electrodes to modify the optical conditions of the liquid crystal located at the crossing point of the electrodes in question, this control voltage switches the liquid crystal into a state in which it is reflective or into a state in which it is transparent. In the state in which it is reflective, the cholesteric liquid crystal has a reflection coefficient substantially equal to 0.5 when the light of a polarisation is reflected, and when the light from the other polarisation passes and is absorbed by the absorbent layer. When the control voltage is applied, the liquid crystal switches into the state in which it is transparent and allows practically all the light which is absorbed by the black layer located on the back face of the cell to pass. The observer situated on the front side of the cell, i.e. on the side through which the incident light penetrates the cell, thus sees a dark display on a light background.
The deposition of the absorbent black layer on the back face of the reflective cells poses problems during the manufacturing of such cells. Indeed, as is known, liquid crystal cells are usually manufactured collectively in batches. These batches include two plates common to all the cells and a network of sealing material connecting the plates and forming the walls of said cells, these walls delimiting cavities intended to be filled with liquid crystal. Each batch is divided into rectilinear strips by glass scratching and breaking techniques, or by sawing along parallel straight lines. As each cell has a filling opening along one edge of strip, the cells are then filled and their openings are sealed, then the strip is divided into individual rectangular cells along straight lines perpendicular to the preceding lines. At this stage, if the contour of the cells has parts deviating from the rectangular shape, such parts are shaped by grinding.
The absorbent black layer is applied over one of the two plates common to all the cells before the batches of cells are divided into individual cells. This absorbent black layer may nonetheless not cover the entire surface of the corresponding plate, since it would be damaged and would disturb the cutting and grinding operations. This manufacturing step could also be effected individually on each cell, but that would make manufacturing more complex and expensive than if it could be performed on a batch of cells. It is for these reasons that the technique currently used consists in selectively depositing absorbent black layers on one of the common plates following, for each cell, the final contour thereof. This solution, which is simpler than if the absorbent layer had to be individually deposited on each cell, remains however complex and requires a high level of precision to execute, which inevitably makes the final cost price of the cells more expensive.
Another recurrent problem encountered with reflective cells concerns the optical losses due to the multiple reflections of the incident light at each interface between two materials of different optical indices. This problem will be better understood upon examining FIG. 1 annexed to the present Patent Application which is a cross-section of a reflective display device according to the prior art.
Designated as a whole by the general reference numeral 1, the aforementioned reflective display device includes a first transparent substrate 2 arranged on a front side, i.e. on the side which receives incident light, and a second substrate 4 arranged on a back side, so that substrate 4 extends facing front substrate 2 and parallel thereto.
Substrates 2 and 4 may be made of glass or a plastic material. They are connected via a sealing frame 6 in order to limit between them a cavity for containing a film or layer 8 of reflective liquid crystals, for example of the cholesteric type.
The surface of substrate 2 turned towards substrate 4 carries a first group of electrodes 10. The surface of substrate 4 turned towards substrate 2 includes a second group of electrodes 12 arranged, for example, in the form of strips perpendicular to electrodes 10. Each of electrodes 10 and 12 may be formed by a conductive strip deposited and structured by conventional techniques and made, for example, of indium-tin oxide (ITO). The display device is connected to a control circuit for electrodes 10 and 12, this circuit supplying control voltages to the electrodes to switch the liquid crystal into a state in which it is reflective or into a state in which it is transparent. Finally, the outer face of back substrate 4 is coated with an absorbent black layer 14.
As can be seen in FIG. 1, the incident light whose intensity is designated by the alphabetical reference I is partially reflected at each interface between two mediums of different optical indices. Thus, the following references will successively be used:
R0 designates the intensity of the light reflected by the external surface of front substrate 2;
R1 designates the intensity of the light reflected at the interface between the back face of front substrate 2 and the first electrode group 10 deposited on this surface;
R2 designates the intensity of the light which is reflected at the interface between the first electrode group 10 and liquid crystal layer 8;
R3 designates the intensity of the light which is reflected at the interface between liquid crystal layer 8 and second electrode group 12;
R4 designates the quantity of light reflected at the interface between second electrode group 12 and the front face of back substrate 4 on which said second electrode group 12 is deposited; and
R5 designates the quantity of light reflected at the interface between the back face of back substrate 4 and absorbent black layer 14 deposited on this back face of said second substrate.
Finally, Rx designates the quantity of light reflected by cholesteric liquid crystal 8.
In order to give an order of magnitude, and ignoring the multiple internal reflections, it is estimated that the reflection intensity R0 represents approximately 4% of the total intensity I of the incident light, that reflections R1, R2, R3 and R4 each represent approximately from 1 to 2% of the total quantity of incident light 1, and that R5 is of the order of 1% of incident light 1. One thus reaches, for reflections R0 to R5, a total of the order of 10% of the quantity of incident light 1, which is far from negligible and leads to an alteration in the luminosity of the characters displayed by display device 1.
Reflections R0, R1, R2, R3, R4 and R5 also have the effect of reducing the display contrast CR. Indeed, display contrast CR is defined as the ratio between the reflectivity R of display device 1 when liquid crystal 8 is switched into the state where it reflects the incident light, and the reflectivity r of display device 1 when liquid crystal 8 is switched into the stage where it is substantially transparent. Reflectivity R is the sum of reflections R0 to R5 increased by the quantity of light Rx=RB reflected by cholesteric liquid crystal 8 when the latter is in the reflective state, and reflectivity r is the sum of reflections R0 to R5 increased by the quantity of light Rx=RS reflected by cholesteric liquid crystal 8 when it is in the transparent state where it allows substantially all the light to pass. Consequently, display contrast CR is determined at a first approximation by the following ratio:       C    ⁢          xe2x80x83        ⁢    R    =            R      r        =                            R          ⁢                      xe2x80x83                    ⁢          B                +        R0        +        R1        +        R2        +        R3        +        R4        +        R5                              R          ⁢                      xe2x80x83                    ⁢          S                +        R0        +        R1        +        R2        +        R3        +        R4        +        R5            
As will easily be understood, the value of display contrast CR is greater, and consequently the characters displayed by display device 1 are better contrasted, the smaller the reflectivity r of said display device 1 in the dark state, i.e. in the state in which cholesteric liquid crystal 8 is practically transparent and only reflects a small amount of light. As a general rule, the quantity of light RS reflected by cholesteric liquid crystal 8 in the transparent state, cannot be reduced by much. However, one can try to reduce reflections R0 to R5 by arranging, at each interface between two mediums of different optical indices, anti-reflecton layers. However, these anti-reflecton layers are usually very expensive and considerably complicate the manufacturing process of the display cells. This is why such anti-reflecton layers are mostly not used and it is accepted that display devices with a mediocre display contrast are put on the market.
The main object of the present invention is to overcome the aforementioned drawbacks, in addition to others, by providing a reflective liquid crystal display device allowing the display of contrasting data which is simple and thus less expensive to manufacture.
According to a first aspect, the present invention therefore concerns a liquid crystal display device including a substrate arranged on a front side, i.e. arranged on the side receiving an incident light, and a second substrate arranged on a back side, so that this second substrate extends facing the front substrate parallel thereto, the first and second substrates being connected via a sealing frame in order to delimit between them a cavity for containing the liquid crystals and including on their opposite faces a first and a second group of electrodes respectively, the display device being connected to a control circuit for the electrodes, this circuit being able to supply suitable control voltages to selected electrodes in order to modify the optical conditions of the liquid crystal at the crossing point of the electrodes in question, i.e. to switch the liquid crystal into a state in which it is reflective or into a state in which it is transparent, said display device further including an absorbent black layer made of an electrically non-conductive material, characterised in that the absorbent layer is arranged at the same level or at a higher level in the stacking order of the substrates than the face of the back substrate which carries the second group of electrodes.
Owing to these features, the present invention provides a display device wherein the quantity of light reflected by parasit reflections is reduced, which allows the display contrast to be substantially improved. Moreover, one is no longer obliged, in order to be able then to scratch the glass during the step of cutting out the display cells, to deposit selectively the absorbent black layer, which simplifies the manufacturing operations and thus contributes to a reduce in the cost price.
According to a complementary feature of the invention, the second group of electrodes is made of a black-coloured electrically conductive material, and is associated with an absorbent layer which is also black and made of an electrically non-conductive material whose reflection coefficient is close or equal to that of the material used to elaborate the electrodes, this layer covering at least the parts which are not covered by the electrodes.
In this way, in the zones where the liquid crystal is switched into the same transparent or reflective state, the light will be absorbed or reflected in an almost identical manner by the black layer and by the electrodes, and will thus provide a very homogenous colour display over the entire surface of said display device. An optimum display contrast is also obtained.
More precisely, by using a black electrode, the parasite reflections which are usually generated at the interface between the liquid crystal layer and the second group of electrodes on the one hand, and at the interface between this same second group of electrodes and the front face of the back substrate on which said second group of electrodes is deposited, are removed. Thus, the electrodes will not contrast with the rest of the display surface of the device according to the invention, so that these electrodes will be totally invisible to the observer. On the other hand, since the second group of electrodes plays both the role of electrode for supplying control voltages switching the liquid crystal from a reflective state to a transparent state and vice versa, and the role of absorbent black layer, the manufacturing step which consists in selectively depositing the absorbent black layer after the deposition and structuring of the electrodes no longer requires any great precision, which, of course, has a favourable impact on the manufacturing costs of the display devices according to the invention.
According to a second aspect, the present invention concerns a liquid crystal display device including a substrate arranged on a front side, i.e. on the side receiving an incident light, and a layer arranged on a back side, so that this layer extends facing the front substrate parallel thereto, the front substrate and the layer carrying on their opposite faces a first and second group of electrodes respectively, a film formed by a plurality of liquid crystal capsules or by a polymer dispersed liquid crystal being arranged between said front substrate and said layer, characterised in that the layer is a black-coloured absorbent layer made of an electrically non-conductive material, and in that the second group of electrodes is made of an electrically conductive material which is also black, the reflection coefficient of the material used to elaborate the absorbent black layer being close or equal to that of the material used to elaborate the electrodes.